Frequency sweep modulator



MOTOR Dec. 30; 1947. MURDOCH JR 2,433,782

- FREQUENCY SWEEP MODULATOR- Filedbct. 2, 1959 -2 Sheets-Sheet 1 PULVSZZPER/OD E6.

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3nventor Gttomeg i I AlexandrMurdoahfi Dec. 30, 1947. A. MURDOCH, JR

FfiEQUENCY SWEEP MODULATOR 2 Sheets-Sheet 2 Filed Oct. 2, 1939 a h? um. r M r. e d m x 6 Z A Patented Dec. 30, 1947 FREQUENCY SWEEP MODULATOR Alexander Murdoch, Jr., Philadelphia, Pa., as-

signor to Radio Corporation of poration of Delaware Application October 2, 1939, Serial No. 297,601

This invention relates to frequency sweep modulators, and especially to adevice for varying the frequency of a pulse signal.

Pulse signals are often used to locate a signal reflecting object. In some systems, the object is located by triangulation; in other systems, the distance of the object is determined by observing the time required for the signal to travel from the transmitter to the reflecting object and back to a suitable receiver. In such distance measuring systems, sharply defined pulses are transmitted at intervals of the order of twice the time required for the pulse to travel the maximum range of the device. In some types of devices, supersonic frequencies are 'used. When the received signals are weak, it is difflcult to distinguish the pulse echo signals from signals caused by reverberations and other interfering sounds. If.the pulse signal frequency is varied, the received signal is heard as a musical note which can be distinguished from the non-musical interference.

The present invention has for one of its objects the provision of means for modulating the frequency of a, pulse. Another object is to provide means for modulating the frequency of a pulse in synchronism with the generation of the pulse. Another object is to provide means for sweeping the frequency of a pulse through a desired range by means of an electronic device.

The invention will be described by reference to the accompanying drawings in which Figure l is a graph illustrating unmcdulated and modulated pulse characteristics, and Figures 2 and 3 are schematic circuit diagrams embodying the invention.

Referring to Fig. 1, in the prior art systems, pulse signals l having a pulse period 3, are transmitted at intervals 5. The reflected or echo signals I are of smaller amplitude. The pulses are composed of a number of waves 9, preferably of supersonic frequency. The wave frequency is constant during the pulse periods as illustrated in the second portion H of the graph. According to the-present invention, the frequency of the waves is increased during each pulse, as shown in the third portion l3 of the graph. The nature of the frequency increase is illustrated in the fourth portion l5 of the graph.

While only a fewwaves have been shown, it should be understood that any desired number of waves may be radiated. Furthermore, the frequency may be increased or decreased, and the rate of frequency change maybe linear or nonlinear, It is preferable to synchronize the he- 6 Claims. (Cl. 179- 1715) America, a corquency sweep with the. beginning of each pulse,

and to have the receiving device responsive to a frequency within the sweep.

Referring to Fig. 2, a sound projector and transducer l! is connected to an oscillator 19 which is keyed by a keying source 2|. The keying source is mechanically operated by a motor ;23. The motor is also used to drive an indicator 25. The indicator is connected to the output of a receiver 21 whose input is connected to the transducer ll. Telephone receivers 29,01- other acoustic'signal devices, are connected to the receiver output.

In the operation of the system, the keying source determines the pulse length and pulse intervals. The oscillator determines the wave frequency. The waves are projectedfrom the sound projector and, after reflection, are received thereon and converted into electric waves, which are received," demodulated and amplified by the receiver 21. Thereceived audio pulses may be heard in the receivers and observed on the indicator, as is known to those skilled in the art.

The sweep modulator is connected as follows: a double pole-double throw relay operated switch 3| is arranged so that variable capacitors 33, are substituted for the fixed capacitors 31, 39 of the oscillator. The capacity ofthe fixed capacitors preferably equals theaverage capacity of the variables. The variable capacitor shaft is connected to a cam 4| and a motor 43. The motor is preferably ofthe split-phase type in which a capacitor 45 is connected in series with one of the windings to provide the phase shift for starting. The rotor is pre'ferably constructed of a permanent magnet material and is arranged to formra large number of permanent field poles.

These poles insure rotation at a "speed synchronous with the rotation of the electrical field of the stator and provide quick starting andstopping of the motor.

The motor connections are asfollowsz one ter minal of each field winding is connected to one terminal of the Al-C. source 41. The other terminals of thefield windings are connected together and to the fixed terminals 49, 5| of switches 53, 55. The'movable terminals 51, 59 are connected, respectively, to'the fixed terminals of a double pole-double throw switch 6|, which is operated by a relay B3. Therelay is connected to the keying source. The'movable terminals of the switch 6| are joined together and connected to the other terminal of the A.-C. source.

The operation of the sweep modulator circuit is as follows: The relay operated switch 3| con- 3 nects the variable capacitors 33, 35 to the oscillator when the power circuit switch 61 is closed. When a keying pulse is applied from the keying source 2|, the relay B3 is closed. The relay 63 connects the A.-C. power source 41 through the switch 53 to the motor 43. The motor starts, almost instantaneously, and drives the variable capacitors throughthe; desired tuningrange, and, also drives the; cam 41in a counter-clockwiseposition. At the end of the keying pulse, the relay 53 will release the movable contacts of switch 6,1,. The movable contacts 65 then connect the power source through the switch 55 which has been closed by the cam 4|. During the keyingpulse the shaft of the variable capacitorshas been ro.-.

tated so that the capacitors have changed from maximum to minimum capacitor. Themotor will.

continue to drive the cam and variable capacitor shaft until the cam and variable capacitorshayej been restored to their initial position, at which position the cam opens switch 55, and stops the meter. At'the. initial position. switch 53 is closed and the sweep modulator-is againready to sweep the, oscillatorfrequency in synchronism with the next keyi g pulse." Thes itchesBQ, I I. in the capacitor circuits 33, 3.5 and. & 39..are used to provide' twq sweep ranges.

Inamodificatiomillustrated in. Fig. 3, the oscillator frequency is varied, by electronic means. The'oscillator 1.3. includes a. grid capacitor 15 and. a resistor 11-. which biases off the oscillator during keying intervals. The keyingv period and keying intervals are determined by motor driven cams l9, 8] The cams, whichare relatively adjustable, drive switches. 83, 8.5.. The switches 83, 85 are connected in series. and, hence, to the resistor 11. The upper camoperated. switch 81,. 85 determines the start of the keying; the lower switch 83 determines the end. of the keying, pulse; and the motor speed determines. the pulserate.

Av battery 8.! is connected to variable resistors 89, 9|. which are connected in parallel. The resisters are connectedto acircuitincluding an inductor 9 3',.resistor 9.5, and capacitor 9].. The connection continues from. the circuitv 93, 95,, 91 through a switch 9,9, operated by the cam 8|, back to the battery 81'. The inductor 93 is positioned on the magnetic core, IOl' which couples the oscillator inductors I 3, 15'. One of the resistors 89 is connected'to thefmotor operating. the cams and is variedin synchronisrn therewith.

The operation'is. as. follows; First, assuming that thebattery 81 is disconnected, the'closin 0f the'first and s e cond .cam operated switches 8|, 83 starts, the oscillatp j- The signal length is determined by the opening. of the second' switch 83'. Now, assuming that the battery. 811 is. connected, currents will flow. thIQI- gh the, inductor 9.31and thus the frequency, of. the oscillator 13 will be varied during the keying pulse. It isdesirable to have the oscillator frequencyvariation the same for short and longpulses'so thatthe oscillator frequency will'always sweep through the same resonant. frequencyw which is generally the frequency of the receiver. The variable resistors 89, 9 l, along with the time. constant of the circuit 9 3, 9 5,, 91;. are adjusted sothat the variation of the oscillator frequency may. be synchronized. with the keyingjpulse, andmay be arranged to cover a. uniform frequency sweep. Inthis connection, itv should be. understood that eq ivalent meansfor varying. the frequencyof; the oscillator or generator arewithimthescopeof the invention. o e. the adjiistment has. been, made, the cam, operated; resistor will, automatically syn- 4 chronize and adjust the sweep frequency to the pulse period.

Thus the invention has been described as a sweep frequency modulator in which the frequency of a. generator or oscillator is varied during a keying pulse of short duration. The variation is preferably synchronized with the keying pulse so that the: sweep covers the same range, and, so. that; the frequency of the oscillator is the same at the beginning and end of each pulse. This mode of operation insures the oscillator frequency passingthrough the resonant frequency of the receiver. It should be understood that the frequency sweepmay be linear or exponential.

lfclaim as. my invention:

1.. A sweep modulator including a keying source, an oscillator. connected thereto, means for varying the frequency of said oscillator, a motor for operating said frequency varying means, means for intiating energization of said motor at the start, of each, keying pulse, and meansincluding said motor for restoringv said. frequency varying means to its initial position after the terminating of said of said keying, pulse andgfor deenergizing; Said motor.

2. A. sweep modulator including a pulse keyingv source, an oscillator. connected. thereto, frequency varying means connected to said; oscillator, a motor for operating, said frequency varyingv means, switches operated;- by said keying source, and additional switches operated by said motor, saidlswitches being so. connectedthat. said motor is energized at the, start of-a keying pulse and, de-energized after the completion. Ofsaid, pulseand after saidfrequency varyingmeans is re-- stored to its initial position.

3. A. sweep frequency modulator including, in

combination, a sourceof oscillations, a, variable capacitor connected tQ Sflid'; source for'varying, the frequency of saidoscillations, keying; means,

for keying; said, source onand off to establish a pulse of said oscillations, a motor for controllingsaid capacitor, switches operated; by said, motor and connected. thereto. arranged to 0116 said,

motor circuit. at, a; predetermined value of said. variable capacitor; and, means,.operated by said keying source for starting; said ,Imotor in, substantial synchronism, with ,said pulse.

4". A sweep frequency modulator including, in,

combination, a, source .of oscillations, av variable capacitor, connected tosaidsource for varying the frequency of, said, oscillations; keying, means for, keying said source. on-and off. iioestablish apulsa of said. oscillations a rapid. self starting, synchronous motorfor controlling said; capacitor, switches operated by said motor and connectedthereto arrangedjto open. said motor, circuit-at a p deter ed; value f. said, variable apacitor, nd mean operated. by, said. key n source for starting said motor in; substantial. synchronism with said; pulse. l I v I 5. A sweep. fre uency-modulator including, inm i t n;as nerator. of alternatin currents. a variable reactior connected to said generator for t ng he, frequencyIof. said currents, keying means for obtaining pulses or saidalternate mined ,position v of said reactor, camsconnectedto a m t ndw acten o ope Said.

h s, nd: mea s; r snonsiva. to. said, keyin means forstarting; th

t yn hronisrnl w thsa d pu ses enoner tion ofzsaidcmotorrin 6. A sweep frequency modulator including, in combination, a generator of oscillatory currents, REFERENCES CITED means effectively connected to said generator for The f ll i references are of record in the varying the frequency of said currents, a keying file of this patent: source connected to said generator for establish- 5 ing pulses of said currents, and means respon- UNITED STATES PATENTS sive to said keying source and effectively con- Number Name 7 Date nected to said generator for varying said fre- 2,011,392 Bentley Aug 13- 1935 quency during each said pulse from an initial value and back again to said value and synchro- 1o nized with said pulse.

ALEXANDER MURDOCH, JR. 

